The present invention relates to additives for enhancing corona stabilization in electronegative gases and, more particularly, relates to methods and gases for improving electrical breakdown voltage characteristics in such insulating gases for divergent field situations.
In the electric utility industry, there have been concerted efforts to increase energy efficiencies in the generation and transmission of electrical energy. Many of the schemes for increasing energy efficiencies have also been associated with need for operation at higher voltage levels. Accordingly, much effort has been expended over the previous decade, and before, to develop gases exhibiting excellent insulating properties, particularly for use in newer, more efficient electrical energy production and transmission systems. A particularly useful insulating gas has been found to be sulfur hexafluoride, SF.sub.6. However, it has also been found that typical electronegative insulating gases such as SF.sub.6 exhibit their optimal divergent field breakdown voltages at pressures which are higher than atmospheric. In particular, it has been generally observed that pressures of from about 1 to 6 atmospheres are desirable for SF.sub.6 itself. It has also generally been observed, with respect to SF.sub.6, that at operating pressure ranges above approximately 3 atmospheres divergent field corona stabilization ceases to provide enhanced insulating properties. In particular, it has been observed that in such pressure ranges, there is a vanishingly small voltage difference between inception of corona discharge and ultimate breakdown of the insulating gas in nonuniform fields.
In an insulating gas, electron collisions with neutral gas molecules typically result in one of two possible conditions. Either the colliding electron will ionize the molecule, producing separate charged entities or the electron will attach itself to the molecule, in which case ionization does not result. The rate at which electron collisions result in ionization is referred to as the ionization coefficient .alpha.. The rate at which electrons attach themselves to the gas molecules is referred to as the attachment coefficient, .eta.. Accordingly, the net ionization coefficient, .alpha., is defined as .alpha.-.eta.. For electronegative gases, .eta.&gt;0. It is these electronegative gases which are generally of concern herein.
In high voltage electrical systems, the greatest problems arise in those areas in which the electric field is nonuniform. Such nonuniform, divergent electric fields most typically are produced in those situations in which there is a sharp-edged feature in the high voltage system under consideration. Such divergent field-causing features may often include such mundane surfaces as the sharp edges on hex head bolts or particulate contamination in systems which are designed to exhibit quasi-uniform fields. It is in these regions in which the greatest electric field strengths are produced and, accordingly, such sharp edges and points are often the initiating points for gas breakdown. In fact, it is comon amongst experimenters in spark breakdown phenomena to simulate such sharp edges through the use of a point electrode spaced a certain distance from a flat planar electrode. Such experimental conditions were employed in investigations involving the methods and mixtures of the present invention. Furthermore, it has also been well documented in the art that the greatest problems associated with electrical breakdown occur in those instances in which the pointed electrode is connected to the positive side of a voltage source.
However, it is well documented that in such divergent field situations as are found in the point/plane gap configuration with the point being positive, that the corona discharge occurring has a stabilizing influence on the breakdown characteristic. Nonetheless, it should be noted that it is also known that this phenomena is highly dependent upon the pressure of the gas employed. For example, for gases such as SF.sub.6, it is known that in the absolute pressure range between about 1 atmosphere and 3 atmospheres, the presence of corona discharge produces a stabilizing influence on the breakdown characteristics of the gas. However, this advantage has in the past not been extendable to higher insulating gas pressure ranges which are of particular importance in the electrical utility industry. The physics contributing to the corona enhancement of breakdown voltage is very complex, but, in general, the onset of corona forms a conducting cloud in the region of the point or sharp edge, which has the effect of modifying the field to increase the breakdown voltage. The collaborative streamers forming a part of this conducting cloud exist only within an ionization zone for which the net ionization coefficient, .alpha., is greater than 0. These collaborative streamers are initiated by secondary electrons produced by photoionization. Accordingly, the number of electrons produced by photoionization is dependent upon photoabsorption in the gas. However, the characterisitic photoabsorption curve for insulating gases has generally been ignored as a means for improving corona stabilization characteristics and the associated increases in breakdown voltage in the insulating gas, particularly in gas pressure ranges of industrial interest.